In radio communications such as mobile telephony system and the like, as a technology for enhancing the transmission speed without widening the frequency bandwidth, MIMO (Multiple Input Multiple Output) transmission for performing spatial multiplexing transmission using a plurality of transmitting and receiving antennas has been known.
FIG. 18 is a block diagram showing a configuration of a transmitting apparatus for MIMO transmission. The transmitting apparatus includes a serial-to-parallel converter 5001, modulators 5002-1 to 5002-T, transmitters 5003-1 to 5003-T and transmitting antennas 5004-1 to 5004-T.
To begin with, a transmission bit sequence is serial-to-parallel converted by serial-to-parallel converter 5001 so as to be divided into T bit sequences. The bit sequences are mapped on modulation symbols such as QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation) by associated modulators 5002-1 to 5002-T. The modulation symbols, as the transmitted signal, are converted into radio frequency through transmitters 5003-1 to 5003-T and transmitted at an identical frequency at the same timing from associated transmitting antennas 5004-1 to 5004-T.
FIG. 19 is a block diagram showing a configuration of a receiving apparatus for MIMO transmission. The receiving apparatus includes receiving antennas 5101-1 to 5101-R, receivers 5102-1 to 5102-R, a signal detector 5103 and a channel estimator 5104.
Receiving antennas 5101-1 to 5101-R receive the transmitted signal in a spatially multiplexed form. The received waves picked up by receiving antennas 5101-1 to 5101-R are converted from the radio frequency to the baseband by respective receivers 5102-1 to 5102-R to be output as received signals. Signal detector 5103 detects the transmitted signal based on the received signal and channel-estimated values obtained from channel estimator 5104 and outputs decision value of the transmission bit sequence. Channel estimator 5104 estimates impulse response of a transmission channel, based on a known training signal for channel estimation and the received signal.
Signal detector 5103 detects individual transmitted signals from the received signal that is given in a spatially multiplexed forms of the transmitted signal. As an optimal detecting scheme, MLD (Maximum Likelihood Detection) is used. First, the received signal is represented as follows:
                    [                  Math          ⁢                                          ⁢          1                ]                                                            y        =                  Hs          +          n                                    (        1        )                                y        =                              [                                                                                y                    1                                                                    Λ                                                                      y                    R                                                                        ]                    T                                    (        2        )                                H        =                  (                                                                      h                  11                                                            Λ                                                              h                                      1                    ⁢                                                                                  ⁢                    T                                                                                                      M                                            O                                            M                                                                                      h                                      R                    ⁢                                                                                  ⁢                    1                                                                              Λ                                                              h                  RT                                                              )                                    (        3        )                                s        =                              [                                                                                s                    1                                                                    Λ                                                                      s                    T                                                                        ]                    T                                    (        4        )                                n        =                              [                                                                                n                    1                                                                    Λ                                                                      n                    R                                                                        ]                    T                                    (        5        )            
Here, y is an R-dimensional received signal vector having as its components the received signals that have been received at individual antennas, H is a R-row, T-column channel matrix having as its elements the channel impulse responses between the transmitting antennas and receiving antennas, s is a T-dimensional transmitted signal vector having as its components the transmitted signals that have been transmitted from individual antennas, and n is a R-dimensional noise vector having as its components noises at individual receiving antennas. The superscript “T” denotes transpose of a matrix. MLD detects the transmitted signal based on the received signal, channel-estimated values and transmitted signal candidates, following the criterion below
                    [                  Math          ⁢                                          ⁢          2                ]                                                                      s          ~                =                  arg          ⁢                                          ⁢                                    min                              s                ^                                      ⁢                                                                            y                  -                                                            H                      ~                                        ⁢                                          s                      ^                                                                                                  2                                                          (        6        )            
Here, s{tilde over ( )} is a detected T-dimensional transmitted signal vector, H{tilde over ( )} is a R-row, T-column channel estimate matrix having as its component estimated channel impulse responses, s^ is the candidates of the transmitted signal. s^ involves all the signal patterns transmitted on the transmitting side. In MLD, based on the transmitted signal candidate that is closest to the received signal or that minimizes the metric:[Math 3]∥y−{tilde over (H)}ŝ∥2,
from all the candidates of the transmitted signal, the transmission bits are determined so as to be output as the decision value.
In this way, since in MLD, as many metrics as the number of all the transmitted signal candidates are calculated, it is possible to obtain the optimal performance, but this method poses a problem that the amount of computing operation becomes huge. Concerning MLD, there is a description in Non-patent Document 1 mentioned hereinbelow.
As the second-to-best detecting scheme that can cut down the amount of computing operation, there exist linear reception schemes such as ZF (Zero Forcing), MMSE (Minimum Mean Square Error), for example. A linear reception scheme is carried out by multiplying the received signal by a T-row, R-column weight coefficient matrix, as follows:[Math 4]{circumflex over (x)}=Wy  (7).
The weight coefficient matrix W is given as[Math 5]W=({tilde over (H)}H{tilde over (H)})−1{tilde over (H)}H  (8)on the ZF basis, and the matrix is given as[Math 6]W=({tilde over (H)}H{tilde over (H)}+σn2IT)−1{tilde over (H)}H  (9)on the MMSE basis.
Here, H represents a complex conjugate transposed matrix, σn2 denotes noise power, IT denotes a T-row, T-column unit matrix. By making a hard decision on x^ in equation (7), it is possible to obtain a transmission bit sequence. In this way, in the linear reception scheme, the number of the candidates of the transmitted signal is substantially narrowed down to one, so that it is possible to sharply cut down the amount of computing operation compared to that in MLD. Concerting the linear reception scheme, a description is found in Non-patent Document 2 mentioned hereinbelow.
However, the linear reception scheme can sharply reduce the amount of computing operation on one hand, but causes noise enhancement on the other hand, posing a problem of degrading reception performance.
In regard to MLD, there is a technique whereby preferable reception performance can be obtained while the candidates of the transmitted signal are narrowed down hence the amount of computing operation is markedly cut down, by searching in the direction of noise enhancement as the direction in which reception performance degrades. This technique is described in Patent Document 1 mentioned below.
FIG. 20 is a block diagram showing a configuration of a signal detector 5103 shown in Patent document 1. Signal detector 5103 in patent document 1 includes a transmitted signal candidate generator 5200, a metric generator 5206 and a minimum metric detector 5207. Transmitted signal candidate generator 5200 includes an initial signal generator 5201, adders 5202-1 to 5202-T, quantizers 5203-1 to 5203-T, parallel-to-serial converter 5204 and an updating value processor 5205. Initial signal candidate generator 5200 generates transmitted signal candidates. Initial signal generator 5201 generates an initial signal by multiplying the received signal by the weight on the ZF or MMSE basis given as equation (8) or equation (9). Adders 5202-1 to 5202-T each add the initial signal and the updating value obtained from updating value processor 5205. The added result is subjected to hard decision at quantizers 5203-1 to 5203-T, then parallel-to-serial converted by parallel-to-serial converter 5204 into a transmitted signal candidate.
Updating value processor 5205 determines updating value from the received signal, the initial signal and channel-estimated values, following the subsequent Eqs. The updating value is assumed to be u, u is determined by[Math 7]u=μrv  (101)v=Pq{tilde over (H)}H(y−{tilde over (H)}ŝ(0))  (102)P=({tilde over (H)}H{tilde over (H)}+σ2IT)−1  (103)μr=[a(m)−({circumflex over (x)})t]/(v)t  (104)
Here, q is an integer equal to or greater than 1, y is the received signal vector, s^(0) is the hard-decision result of the initial signal. (x^)t and (v)t are the t-th elements of x^ and v, respectively. a(m) may take M values for each t; M the multiple-valued number for modulation. Since a transmitted signal candidate is obtained by adding updating value to the initial signal and then subjecting the sum to hard decision, as many candidates as the number of μr, i.e., TM candidates are obtained. The obtained TM candidates are subjected to maximum likelihood detection. In the conventional MLD, metric calculations for MT candidates were performed. In contrast to this, the number of transmitted signal candidates can be sharply reduced. In addition, since transmitted signal candidates are searched for, by taking noise enhancement into account, it is possible to pick up candidates close to the actual transmitted signal. Accordingly, it is possible to inhibit degradation of reception performance while sharply cutting down the amount of computing operation.
Patent Document 1:
    Japanese Patent Application Laid-open 2007-300586Non-Patent Document 1:    X. Zhu and R. D. Murch, “Performance analysis of maximum likelihood detection in a MIMO antenna system, “IEEE Transaction on Communications, vol. 50, no. 2, pp. 187-191, February 2002.Non-Patent Document 2:    Simon Haykin, Adaptive Filter Theory The Third Edition, published by Prentice-Hall 1996.